Technical Field
The disclosure generally relates to liquid crystal displays.
Description of the Related Art
Liquid crystal displays (LCDs) are widely used in electronic devices, such as laptops, smart phones, digital cameras, billboard-type displays, and high-definition televisions.
LCD panels may be configured as disclosed, for example, in Wu et al., U.S. Pat. No. 6,956,631, which is assigned to AU Optronics Corp., the parent company of the assignee of the current application, and hereby incorporated by reference in its entirety. As disclosed in Wu et al. FIG. 1, the LCD panel may comprise a top polarizer, a lower polarizer, a liquid crystal cell, and a back light. Light from the back light passes through the lower polarizer, through the liquid crystal cell, and then through the top polarizer. As further disclosed in Wu et al. FIG. 1, the liquid crystal cell may comprise a lower glass substrate and an upper substrate containing color filters. A plurality of pixels comprising thin film transistor (TFT) devices may be formed in an array on the glass substrate, and a liquid crystal compound may be filled into the space between the glass substrate and the color filter forming a layer of liquid crystal material.
As explained in Sawasaki et al., U.S. Pat. No. 7,557,895, which is assigned to AU Optronics Corp., the parent company of the assignee of the current application, and hereby incorporated by reference in its entirety, the thickness of the liquid crystal layer typically must be uniformly controlled, in order to avoid unevenness in brightness across the LCD panel. As disclosed in Sawasaki et al., the required uniformity may be achieved by disposing a plurality of pillar spacers between the TFT substrate and the color filter substrate. As further disclosed in Sawasaki et al., the pillar spacers may be formed with different heights, such that some spacers have a height that is greater than the gap between the substrates and other spacers have a height that is less than the gap between the substrates. This configuration may permit the spacing between the substrates to vary with temperature changes but also prevent excessive deformation when forces are applied to the panel.
Sawasaki et al. further discloses a method for assembling the substrates with the liquid crystal material between them. This method comprises steps of preparing the two substrates, coating a sealing material on the circumference of the outer periphery of one of the pair of substrates, dropping an appropriate volume of liquid crystal on one of the pair of substrates, and filling in the liquid crystal between the pair of substrates by attaching the pair of substrates in a vacuum followed by returning the attached pair of substrates to atmospheric pressure.
In LCD panels, the semiconductor material making up the TFT channel may be amorphous silicon. However, as disclosed in Chen, U.S. Pat. No. 6,818,967, which is assigned to AU Optronics Corp., the parent company of the assignee of the current application, and hereby incorporated by reference in its entirety, poly-silicon channel TFTs offer advantages over amorphous silicon TFTs, including lower power and greater electron migration rates. Poly-silicon may be formed by converting amorphous silicon to poly-silicon via a laser crystallization or laser annealing technique. Use of the laser permits fabrication to occur at temperatures below 600° C., and the fabricating technique is thus called low temperature poly-silicon (LTPS). As disclosed in Chen, the re-crystallization process of LTPS results in the formation of mounds on the surface of the poly-silicon layer, and these mounds impact the current characteristics of the LTPS TFT. Chen discloses a method to reduce the size of the LTPS surface mounds, by performing a first anneal treatment, then performing a surface etching treatment, for example using a solution of hydrofluoric acid, and then performing a second anneal treatment. The resulting LTPS surface has mounds with a height/width ratio of less than 0.2. A gate isolation layer, gate, dielectric layer, and source and drain metal layers can then be deposited above the LTPS layer to form a complete LTPS TFT.
As disclosed in Sun et al., U.S. Pat. No. 8,115,209, which is assigned to AU Optronics Corp., the parent company of the assignee of the current application, and hereby incorporated by reference in its entirety, a disadvantage of LTPS TFTs compared to amorphous silicon TFTs is a relatively large leakage current during TFT turn off. Use of multiple gates reduces leakage current, and Sun et al. discloses a number of different multi-gate structures for a polycrystalline silicon TFT, including those shown in Sun et al. FIGS. 2A-2B and 3-6.
As is well-known in the art, commonly-used liquid crystal molecules exhibit dielectric anisotropy and conductive anisotropy. As a result, the molecular orientation of liquid crystals can be shifted under an external electric field. By varying the strength of the external electric field, the brightness of the light that passes through the polarizers and the liquid crystal material can be controlled. By applying different electric fields within different pixels of the array, and by providing different color filters for different pixels, the brightness and color of the light passing through each point in the LCD panel can be controlled, and a desired image formed. Such LCDs employ a variety of liquid crystal (LC) mixtures that have been developed to exhibit a range of operating and performance characteristics.
For instance, polymer stabilized blue phase liquid crystal (PS-BPLC) is attractive for use in displays due to some revolutionary features, e.g., no need for an alignment layer, fast response time, and an isotropic dark state. However, PS-BPLC generally requires a high operation voltage because of its relatively rigid polymer network.
From a materials perspective, large dielectric anisotropy (Δε) LC mixtures (e.g., Δε>50) have been developed and employed to generate a large Kerr constant, with a correspondingly lower operation voltage. However, these LC mixtures exhibit a long molecular conjugation length and large dipole moment, resulting in a very high viscosity. Meanwhile, the dielectric constant of BPLC host follows the Debye relaxation:
                              Δɛ          =                                    Δɛ              ∞                        +                                                            Δɛ                  0                                -                                  Δɛ                  ∞                                                            1                +                                                      (                                          f                      /                                              f                        r                                                              )                                    2                                                                    ,                            (        1        )            in which fr is the relaxation frequency and is related to the rotational viscosity η and molecule length l as:
                              f          r                =                              1                          η              ⁢                                                          ⁢                              l                3                                              .                                    (        2        )            
Due to the very high viscosity and long molecular length, the relaxation frequency of high Δε BPLC host is quite low. Unfortunately, such a low relaxation frequency may bring two unwanted challenges: 1) insufficient charging time, and 2) high temperature sensitivity. The challenge of insufficient charging time may be addressed by some novel circuit designs, several of which are disclosed in various publications, such as: C.-D. Tu, et al. J. Display Technol. 9(1), 3 (2013); C.-L. Lin, et al. IEEE Electron Device Letter, 36(4), 354 (2015); C.-L. Lin, et al. US Patent Publication No. 2015/0262542 A1; and, C.-L. Lin, et al. US Patent Publication No. 2015/0277177 A1, for example. However, little progress has been achieved in addressing the issue of temperature sensitivity.
As mentioned above, for a large-Δε BPLC, the Debye relaxation frequency is as low as several kHz. Hence, the Kerr constant strongly depends on the working temperature and driving frequency [F. Peng, et al. J. Mater. Chem. C, 2, 3597 (2014)]:
                              K          =                      A            ⁢                                          exp                ⁡                                  [                                                                                    E                        1                                                                    k                        B                                                              ⁢                                          (                                                                        1                          T                                                -                                                  1                                                      T                            c                                                                                              )                                                        ]                                                            1                +                                                                            (                                              f                        /                                                  f                          0                                                                    )                                        2                                    ⁢                                      exp                    ⁡                                          (                                                                                                    E                            2                                                    /                                                      k                            B                                                                          ⁢                        T                                            )                                                                                                          ,                            (        3        )            where K is the Kerr constant, A is the proportionality constant, kB is the Boltzmann constant, and Tc is the clearing temperature.
As can be seen from FIG. 1, the Kerr constant of a typical large Δε BPLC host BP07 (Δε˜300) decreases from 27.5 nm/V2 to 15 nm/V2, when the temperature increases from 10° C. to 30° C. In such a narrow temperature interval, the Kerr constant changes by approximately a factor of two, and may lead to dysfunction of a display in which the LC mixture is used.
Accordingly, there is a desire to reduce the temperature sensitivity and widen the working temperature range of large Δε LC mixtures, such as PS-BPLC.